A radiation position detector typically includes an array of scintillator elements (such as crystals) and a light receiving element optically coupled thereto. To enhance the spatial resolution of a PET device, the sectional sizes of the scintillator elements with respect to the incident surface of the radiations need to be reduced. The image quality of the PET device therefore depends largely on the sectional sizes of the scintillator elements.
Commercial PET devices include scintillator elements having a sectional size of approximately 4 mm square. As shown in FIG. 1, an arrayed scintillator includes an optical reflective material (hereinafter, may be referred to simply as reflective material) 26 interposed between scintillator elements (detection elements) 20 to control light occurring from interaction with gamma rays. The crystals are discriminated on a barycentric map 35 generated from outputs of a light receiving element 30 by an Anger calculation.
As shown in FIG. 2, to detect a pair of annihilation radiations (also referred to as a line of response) 12 and 14 emitted from inside a body 10 in opposite directions by PET detectors 16 and 18 with high probability, scintillator elements having a thickness of approximately 2 to 3 cm are needed. However, such a thickness can degrade the spatial resolution with respect to obliquely-incident annihilation radiations. Some PET devices then use a three-dimensional position detector capable of detecting a depth of interaction position (Japanese Patent Application Laid-Open No. Hei. 11-142523 (hereinafter, Patent Literature 1), Japanese Patent Application Laid-Open No. Hei. 11-142524 (hereinafter, Patent Literature 2), and Japanese Patent Application Laid-Open No. 2004-279057 (hereinafter, Patent Literature 3)).
There have been proposed various methods for identifying a scintillator crystal of a radiation three-dimensional position detector. One of the most common methods for a layered three-dimensional position detector including three-dimensionally stacked scintillator crystals is one in which two types of scintillator arrays having respective different characteristics are stacked and depth information is identified by waveform analysis (M. Schmand, L. Eriksson, M. E. Casey, M. S. Andreaco, C. Melcher, K. Wienhard, G. Flugge, and R. Nutt, “Performance results of a new DOI detector block for a High Resolution PET-LSO Research Tomograph HRRT,” IEEE Trans. Nucl. Sci., vol. 45, no. 6, pp. 3000-3006, 1998 (hereinafter, Non-Patent Literature 1)).
As shown in FIG. 3, layered three-dimensional radiation position detectors (Japanese Patent No. 5011590 (hereinafter, Patent Literature 4) and T. Tsuda, H. Murayama, K. Kitamura, T. Yamaya, E. Yoshida, T. Omura, H. Kawai, N. Inadama, and N. Orita, “A Four-Layer Depth of Interaction Detector Block for Small Animal PET,” IEEE Trans. Nucl. Sci., vol. 51, no. 5, pp. 2537-2542, 2004 (hereinafter, Non-Patent Literature 2)) have been proposed which detect depth position information by controlling a distribution of scintillation light by an optical reflective material (may be referred to simply as reflective material) 26 and an optical adhesive 28 so that responses of three-dimensionally stacked scintillator arrays (in FIG. 3, four layers) 21 to 24 are projected on a barycentric map 35 without overlap.
To reduce manufacturing cost, a method for machining the interior of the scintillators by using a laser beam (T. Moriya, K. Fukumitsu, T. Sakai, S. Ohsuka, T. Okamoto, H. Takahashi, M. Watanabe, and T. Yamashita, “Development of PET Detectors Using Monolithic Scintillation Crystals Processed With Sub-Surface Laser Engraving Technique,” IEEE Trans. Nucl. Sci., vol. 57, no. 5, pp. 2455-2459, 2010 (hereinafter, Non-Patent Literature 3)) and a method for identifying an interaction position from a distribution of scintillator light on a light receiving surface by using a non-arrayed single scintillator block (F. Sanchez, L. Moliner, C. Correcher, A. Gonzalez, A. Orero, M. Carles, A. Soriano, M. J. Rodriguez-Alvarez, L. A. Medina, F. Mora, and J. M. Benlloch, “Small animal PET scanner based on monolithic LYSO crystals: Performance evaluation,” Med. Phys., vol. 39, p. 643, n/a 2012 (hereinafter, Non-Patent Literature 4)) have also been proposed.